It is known that when an intervertebral disc degenerates or is damaged, there is often a compression of the disc and a reduction in the normal intervertebral height. Typically, this condition results in abnormal motions that become a source of pain.
In order to treat pathologies of this type, the disc is often stabilized to eliminate the abnormal motions caused by disc disorders or injuries. Generally, one approach is to prevent articulation between the two vertebrae situated on each side of the damaged disc by bone fusion. This fusion fixes the vertebrae to each other, eliminating the relative mobility causing the pain. Various spinal implants to promote fusion between adjacent vertebrae have been proposed. It has been proposed to interconnect the two vertebrae by a kind of rigid U-shaped stirrup, which restores the discal height with a bone graft material disposed inside the stirrup. However, one drawback of this proposal is its diminishing effectiveness over a period of time.
Another proposal for promoting spinal fusion includes implanting a spinal cage to interconnect the adjacent vertebrae; the spinal cage includes a cylindrical member provided with a series of openings and provided with anchoring points. This implant is placed in a recess formed in the intervertebral disc and penetrates the opposite cortical endplates of the two vertebrae, which were previously hollowed out to receive the implant. This penetration forms openings in the sub-chondral endplates to place spongy bone of the vertebrae in contact with bone graft material placed inside the implant, facilitating bone fusion. U.S. Pat. No. 5,015,247 provides one example of this approach.
Yet another proposal for spinal fusion comprises inserting hollow tubular implants having a generally ovoidal external shape into the intervertebral space. However, these implants require both annular ribs to inhibit axial displacement and longitudinal ribs or teeth to prevent rotation of the implant about its longitudinal axis. One example of this approach is found in U.S. Pat. No. 5,683,463 issued to Godefroy et al. In another example in U.S. Pat. No. 5,888,224 issued to Beckers et al., a rotatable implant for spinal fusion is disclosed. The rotatable implant requires a linking connector to inhibit longitudinal rotation. Other rotatable implants are described in U.S. Pat. No. 5,607,424 issued to Tropiano.
However, one drawback of these proposed implants is their lack of support of the cortical bone tissue, particularly bearing against the peripheral wall of the vertebral bodies. This contributes to their diminishing effectiveness in maintaining normal disc height over a period of time.
Proper performance of a spinal implant of this type requires balancing the need to promote fusion between the spongy bone and the need to form a reliable load bearing relationship with the stronger cortical bone. As a result, the spinal implant must be neither engaged too far into the openings provided in the cortical endplates to provide a sufficiently dense load bearing surface, nor insufficiently inserted, in which case the bone fusion between the two vertebrae would be adversely affected by a poor anchorage. Thus, there is a demand for devices and techniques that facilitate attaining the proper balance between fusion and load support.
Thus, in light of the above described problems, there is a continuing need for advancements in the treatment of spinal deformities, including improved spinal implants and devices relating to spinal fusion and for surgical methods to treat spinal deformities. The present invention is such an advancement and provides a wide variety of benefits and advantages.